To the OP:
If your steward knows ANYTHING then he'll argue that they can't prove it was you that missed the scans. Fact is they can't prove it was any of the other 3 either. So you're all home free. Feel better?
Whenever you get a warning letter you should file a grievance. Your steward should know this too. Basically all management can show is that there were some missed scans. Unless you were the only person in there which was not the case so you're fine. All that said, just do the job and yup them to death. When you throw a tantrum like you described because you don't wanna get disciplined, you make the target bigger. Chicken legs isn't wrong. Pick your battles wisely. Making a scene isn't in your best interest, but filing on a bogus warning letter is.